Astrophysics

Black holes

In previous article we tried to understand more about neutron stars, but the subject of this article is about even more massive and even denser stellar objects: Black holes. Two years ago, the first ever picture of a black holes was released, which engaged the whole world.

IMAGE 1: First ever photo of a black hole.

In this article we are going to try to comprehend some properties of such dense cosmic formations which black holes are. Note: There are actually three main kinds of black holes, but we are mainly going to look at oneof them in this article: Stellar black holes; and I am consistently referring to them as just a “black holes”. However, a paragraph at the end is designated to each of the two others.

What is a black hole?

Most people are perhaps acquainted with the main concept of black holes, which is that nothing, not even light, can escape it from a certain proximity to it. The property “black”, simply means we do not receive any light from it. This is a rather bizarre concept, but as Einstein’s theory of general relativity suggests, space-time gets warped around massive objects which is what we feel as gravity. Similar to how a trampoline cloth gets twisted when you jump on it, massive objects distort space-time. The heavier the object, the more distorted do the spacetime get, just like a heavier person will make the trampoline cloth bend more pronounced than if a lighter person jumped on it.

IMAGE 2: Illustration of how the curvature of space-time around massive objects. The heavier the object, the more distorted space-time gets.

At the center of a black hole there is a singularity. A singularity is decided by huge amounts of matter being squeezed into an infinitely small region of space, a one-dimensional point. In a singularity the density and curvature of space-time is infinite, and the, until now known, laws of physics cease to be obeyed.

Also, outside the singularity there exists a lot of matter within a relatively small area. As mentioned, from within a certain vicinity to a black hole nothing can escape from it, not even light. The periphery determining whether or not you are to get a black hole getaway, is called the event horizon.

Previous article explained how gravity has something to do with the escape velocity of a neutron star. This regards everything that has a gravitational field, in other words: Everything with mass. For a black hole, which is much denser, the escape velocity exceeds the speed of light. The speed of light is the universal speed limit, meaning nothing can ever travel faster than it. In order for something to escape from a black hole, it would thus need to attain infinite energy, which is impossible. The universe does simply not hold infinite energy!

How are black holes created?

Similar to how a neutron star is created, a black hole is formed as a massive star exhausts its nuclear fuel and implodes due to gravitational collapse. The collapse triggers a supernova explosion, bursting the outer layers of the star away from the core. If the mass of the remnant compressed core is greater than 2.5 solar masses (2.5Mo), it is destined to end is life as a black hole. This was Karl Schwarzschild’s solution to Einstein’s field equations. No force, at least not known to us, is capable of stopping such a star from ultimately collapsing into a black hole.

Once created, black holes can increase in mass and size by accreting matter that approaches it. Even massive neighboring stars and other black holes can be engulfed by a black hole. I think that it is also mentionable that most objects are perfectly fine with simply orbiting a black hole. That is because gravity only supplies the necessary centripetal force so that the object will continue to move in a circular orbit.

How can we see black holes if they are “black”?

We cannot really see a black hole, because they trap electromagnetic radiation, which is what we can perceive with our telescopes. However, we can study how they and their gravity affects their surroundings. For instance, we can see an accretion disc if the black hole was to pass through an interstellar cloud. The black hole will accelerate gas, dust and debris that drift closely, heating it up to millions of degrees. Thus, it glows in X-rays as it is pulled toward the black hole.

We can detect black holes with a technique called gravitational lensing. Basically, because gravity can bend light and as gravity is strong around black holes, the holes can work as lenses. We can measure how much light from distant stars bend as the light passes through a more nearby black hole. Depending on how much the picture of the distant star is distorted viewed through the black hole, one can reckon that there does indeed have to be a black hole in front of the star. Also, recent detections of gravitational waves by LIGO (Laser Interferometer Gravitational-wave observatory) in the U.S. also hopefully opens up for further research on black holes and gravity as well.

IMAGE 3: A rotating black hole and its accretion disc.

For distant observers, only regions outside the event horizon are to be perceived. However, the reality of an object falling into a black hole would be completely different to the former. One’s perception of time and space would change completely. According to the theory of general relativity, the event horizon has no locally detectable features. When something begins to travel with relativistic speeds, or exists in vicinity to a very massive object, time slows down. As light falls into a black hole it gets stretched out, due to gravitational redshift. When it enters the event horizon, the light asymptotically approaches complete redshift, making it fade away entirely and “freeze” time. So, an outside observer would never be able to witness an event horizon to form.

Can nothing really escape a black hole? One might get the impression that black holes can be thought of as some sort of cosmic vacuum-cleaners, as they devour everything that approaches it. This is rather a notorious misconception of black holes. It is only partially true that nothing cannot escape the event horizon of a black hole. Although black holes consume up all the matter in their surroundings, astronomers have also spotted powerful jets of particles propelled from black holes.

While the matter that has entered the event horizon cannot be seen, material swirling outside this threshold is accelerated to millions of degrees. The immense tidal forces in the vicinity of a black hole cause these nearby particles to heat up to millions of degrees and emit high-energy radiation. Matter even closer to the black hole end up getting hurled out into space as jets of particles moving at near the speed of light, along the rotation axis of the black hole. Instead of all the matter approaching the event horizon end up getting trapped, a fraction of lucky particles attains very high speeds and diverts out in narrow energetic beams.

IMAGE 4: The activity surrounding a black hole.

Stephen Hawking also predicted the existence of Hawking radiation. This type of radiation is emitted from black bodies due to quantum effects in vicinity to the event horizon of the black hole. Hawking radiation occurs because empty space, or vacuum, is not really completely empty. There are so-called vacuum fluctuations that allow pairs of particles to pop in and out of existence. If such a pair of particles were to be created near a black hole, there is a chance that one of them could escape the black hole while the other one gets trapped into the black hole. The energy that allows for such pairs of particles to form comes from the black hole itself, so if one of the particles were to escape, the black hole would lose a tiny bit of energy. With time, the black hole will lose energy, or mass in this process.

This theory suggests that, if a black hole is left alone for a long enough time, it will evaporate completely. For a black hole of one solar mass it would take 1067 years to evaporate due to Hawking Radiation. In other words, black holes are going to be around for a very long time. Or at least, it is not Hawking radiation that will be the cause of a black hole’s end.

The two other types of black holes

As mentioned earlier, there are two other types of black holes out there. First, we have primordial black holes which are extremely small and believed to have formed in the infant universe, very soon after the big bang. Scientists think they are as small as a single atom yet have the mass of a large mountain.

The third type of black holes are called “supermassive” and are indeed supermassive. They have masses superior to the mass of about one million suns and fit inside something like our solar system. Scientific evidence suggests that there is a supermassive black hole at the center of every large galaxy. There is, for instance, a supermassive black hole at the center of the Milky Way Galaxy and it is called Sagittarius A. The mass of Sagittarius A is equivalent to about four million solar masses and it would fit into a ball with the same radius as the sun.